I. Field of the Invention
The present invention relates to cellular radio, and in particular, although not exclusively, to a cellular radio system having overlapping macrocells and microcells.
II. Related Art and Other Considerations
A conventional cellular radio system has a number of radio base stations each serving a respective radio coverage area or cell, with which a mobile radio station can communicate over a radio link. As a mobile station moves from one cell to the next, the communication link is transferred from the present base station to the next base station using a procedure known as hand-over or hand-off. The need for hand-over is usually determined on the basis of one or more criteria. Commonly used criteria are:
1) received signal strength indication (RSSI) of the mobile at the base station, or base stations at a mobile station,
2) relative distance measurement of the mobile from the two closest base stations,
3) level of interference from the nearest base station operating on the same frequency, and
4) in digital systems, bit error rate (BER).
The cells of conventional cellular radio systems are relatively large, typically several kilometers across, and this allows time for data to be acquired for even a fast moving mobile station and a decision made on the basis of trends in that data. Recently there have been moves towards having cellular radio systems with relatively small cells of up to a few kilometers in diameter, and typically less than one kilometer in diameter, and these are often referred to as "microcells" while the relatively large, sometimes called wide area cells are often referred to as "macrocells". These terms do not indicate absolute size limitations but rather reflect the relative size of these two cell types. Microcellular radio systems should provide a better frequency re-use and hence greater user density. Proposals for microcellular systems suggest their application to road network, for example motorways where high speed mobile stations will pass through a microcell very quickly. This means that the time available for measurement of a handover criterion, e.g. RSSI, BER or interference level, for use in a hand-over decision is limited. Furthermore, the radio coverage of microcells is subject to large variations in signal strength over short distances relatively, for example in an urban environment.
Taking RSSI as an example of a possible handover criterion in the handover decision or cell reselection for an idle mobile, its use in conventional cellular radio systems is complicated by the variation of RSSI, caused by factors other than pathloss due to changing distance of the mobile from a base station. Considering a simplified situation, see inset sketch in FIG. 1, where a mobile station MS moves between two cells served respectively by base stations BS1 and BS2, the received signal strength of the two base stations at the mobile station varies not only with distance, i.e. diminishes due to path loss, but also varies with fast (Rayleigh) fading and shadowing, see graph in FIG. 1. In this simplified example the `ideal` point for handover is half way between the base station where the RSSI for both base stations is the same--where their pathloss curves intersect. However the effects of fading and shadowing lead to uncertainty of the level of RSSI and to pathloss, hence render determination of the correct handover point more difficult. However by time-averaging the RSSIs the effect of fading and shadowing can to some extent be overcome. This time averaging does introduce a delay, referred to as the time averaging delay, into the handover/cell reselection process. The situation is further complicated, however, because if handover occurs where the averaged values for RSSI are the same there is a significant probability that variation in the momentary levels will be sufficient to trigger the system to make an unnecessary handover back to the original base station. To reduce the probability of such unnecessary handover occuring a hysterasis quantity is introduced which, in effect offsets the RSSI of the current base station relative to the RSSI of destination base station. A simplified example of this is shown in FIG. 2, in relation to the idealised situation of FIG. 1, in FIG. 2 the path loss curve for BS1 is shown offset by a hysteresis element H, with the result that the intersection (handover) point with the path loss curve (2) for BS2 is also offset, in positional terms towards BS2.
The two handover parameters, Averaging period (T) and Hysteris margin H, are related to the two handover qualities frequency of unnecessary handover, and handover delay D as shown in FIG. 3. The standard deviation (S.sigma.) of RSSIs varies with the Averaging period T; the hysteresis margin H is a function of S.sigma., and frequency of probability of unnecessary handover Pu; and Handover delay D is the sum of the Averaging Delay and Hysteris delay. For macrocells of a few kilometers diameter where the Handover Delay in distance terms is of the order of several hundred meters e.g. 700-800 meters, the Averaging period T and Hysteris margin H could typically be T=10 secs and H=7 dB. Such criterion can provide satisfactory results in a macrocell environment, however if applied to a microcell context problems occur.
In a microcellular environment large variations of signal level, and hence RSSI of a base station at a mobile station, occur over relatively short distances, as shown by the example graph in FIG. 4. As indicated in the inset sketch in FIG. 4, microcells A+B overlap at a road junction, and generally provide coverage over only their respective road. When a mobile MS moves from the road served by base station A into the road served by base station B, as shown in the inset sketch in FIG. 4, the mobile MS experiences the variation in signal strength from the base stations of the graph in FIG. 4. The significant effect occurs as the mobile turns the corner, as indicated on the graph, where an almost step function pathloss effect occurs. The present invention is concerned with a cellular radio system which seeks to cope with such microcellular situations while still maintaining the quality of handover control in macrocellular situations.